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Old 04-17-2017, 12:12 PM   #1
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air pressure in your tires

I have a 2004 thomas on a freightliner chassis 40 capacity or so. Here is my forum of my work on the rig.
http://www.skoolie.net/forums/f27/hazys-bus-12678.html
I am curious what you guys use to determine air pressure. A friend told me that the tires don't have to be at their max (115psi) because the bus isn't running heavy (i weigh 18000 lbs.). Right now I have the tires at 95 psi but was wondering if anyone knows a calculation I could use to figure out what psi they should be at for the weight I am running.

The person told me to go lower to give a more comfortable ride but I don't want to do damage to my brand new tires. Thoughts? What air pressure are you all running in your bus?

I still love my bus. Its awesome!
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Old 04-17-2017, 12:16 PM   #2
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I always run tires a few psi (~5) below their "max" rating on the side to allow for expansion as they heat up. You don't want to go much below that though, underinflation causes excess heat which is the #1 cause of tire failure. You should check your pressure regularly, and after any significant temperature change.
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Old 04-17-2017, 12:42 PM   #3
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I guess thats my question still though. The tires say max pressure 115psi (5300lbs) which means that at 115psi my busses tires should hold 30,000 lbs or so but since I am only loaded to 18,000 lbs can I go 2/3 that psi/lb rating that is on the tires (to give a little bounce.) Is it proportional? Is their negatives to dropping the psi on the tires since the bus is no longer carrying the weight it once did?

Extra heat build up in the tires should not be an issue as the weight on my bus is much less? I think? I have only had my bus for a year and a half. Last summers trip I ran the tires at 100psi but have dropped them to 90 psi but my fried says I can drop them more but looking for you guys experience. Thats my question.
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Old 04-17-2017, 12:48 PM   #4
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keep trying and report your findings!

a number of people here run lower tire pressure to soften the ride.

check the tire manufacture's webpage for that tire's info. they probably have what you are looking for.

I've run at 90 and 110. i can't tell any difference, am back at 110 now for sake of mileage (in my head).

good luck
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Old 04-17-2017, 01:40 PM   #5
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What are you trying to accomplish by running a lower tire pressure?
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Old 04-17-2017, 01:43 PM   #6
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I have been told it will "soften" the ride of the bus. I don't see a difference from 110 to 90 but want to know you guys experience.
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Old 04-17-2017, 01:46 PM   #7
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Friction increases as you lower your tire pressure, however it can make a difference in your ride quality. In a lighter vehicle I can easily feel the difference from dropping tire pressure. My bus is also 17 or 18 k for weight while empty and there doesn't seem to be any notable difference in ride quality from lowering the tire pressure. I'd rather take good care of my rather expensive tires.
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Old 04-17-2017, 02:58 PM   #8
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What are you trying to accomplish by running a lower tire pressure?
It can be used to fine tune the ride and tire life.
If you run at max pressure the middle of the tire will wear too fast.
THere are ways of calculating the proper pressure for heavy truck tires.
My bus doesn't have the same size wheels or tires it left the factory with. The School went from little 19.5's to 22.5's with 10R's. It came from the school with 85 psi in every tire.
I'm still playing around with different pressures myself. It seems to like 95 in front and 90 rear, but I'm still messing with getting it dialed in just right.
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Old 04-17-2017, 06:35 PM   #9
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It can be used to fine tune the ride and tire life.
If you run at max pressure the middle of the tire will wear too fast.
THere are ways of calculating the proper pressure for heavy truck tires.
My bus doesn't have the same size wheels or tires it left the factory with. The School went from little 19.5's to 22.5's with 10R's. It came from the school with 85 psi in every tire.
I'm still playing around with different pressures myself. It seems to like 95 in front and 90 rear, but I'm still messing with getting it dialed in just right.
Just make sure you don't air down enough to allow the rears to touch. That is a recipe for a blowout​.

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Old 04-17-2017, 06:50 PM   #10
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That's right. It creates heat until your tire blows. A rock stuck between the duals will do the same thing.
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Old 04-17-2017, 06:50 PM   #11
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Just make sure you don't air down enough to allow the rears to touch. That is a recipe for a blowout​.

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Nah, dude, nothing like that at all not even close.
But most tires say max psi 115 and folks will run that.
If you can figure out the corner weight of the bus, then you can actually find out what the perfect air pressure is for all the tires. Or guess and err on the side of caution.
For the record- AFAIK no one here is talking about running any dangerous psi's or anything. But some of us have to figure out what psi to go with.
My placcard says 19.5's with 85 front 110 rear. IDK what to do with the 10R22.5's its got, so I've been running it the way the school had it.
Started today with 85 on all six. went to 90 on the rears and 95 on the fronts and its riding a little better, at least I think. But the placebo effect is a very real, documented thing, too. lol
I'm gonna sit down some time and actually do all the math and stuff to figure it all out but till then I'm gonna play with em a bit, I think.
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Old 04-17-2017, 08:18 PM   #12
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air changing can be tough... esp if you do it within the same day due to temperature..

for instance here in Cbus its gonna be 45 tonight and then 75 tomorrow and sunny...

at 2% per 10 degrees F.. I could start with a tire that has 100 in the AM then has 106 by afternoon... then I drive the bus and heat it up to a tire temp of say 105 on the strong spring-sun drenched asphalt.. now im at 112... or a little higher since we deal in %... 12 PSI is enough to change the ride quality.... and if I was adjusting in the middle of the day without regards to temperature you can see where that goes.... I could quickly have an under-inflated 'COLD' tire right easily....

the pavement temperatures in Florida this time of year vary wildly also with somewhat cool air in place at night and that very stromng sun by day.
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Old 04-17-2017, 08:29 PM   #13
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Soooooo... does that mean we err on the high side or the low side for safety?
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Old 04-17-2017, 08:38 PM   #14
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I always ran 10psi below max pressure. I could see going up to 20psi if I knew I had a fixed weight.
My Jeep calls for 35psi.. my tires are rated at 80psi max... I run my Jeep at 35 psi.
I am following this thread with interest. Lol

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Old 04-17-2017, 08:47 PM   #15
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I run 85psi, in all tires, was running 100psi. I got the weight of my bus, and looked at a tire chart, at 85psi the ride is softer, and no notable mpg on the scan gauge.
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Old 04-17-2017, 09:18 PM   #16
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These bus tire pressures are set for maximum load conditions while performing the job it was designed for. While I was annoying the tire guys with questions they said some people go as low as 80 psi, but that it's not safe to go lower on a vehicle of this size (18,000 lbs in my case).

My tires are still at OEM pressures. I'm averaging about 2,000 miles a year during this build process. The conditions of trips have been to varied to mess with tire pressures so far.

I've experimented with tire pressures in 4x4s over a period of years. On blacktop roads the higher (maximum recommended) pressures are definitely much rougher riding, but higher pressures do get you better gas mileage. For off road and sand you need progressively lower pressures.
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Old 04-17-2017, 09:27 PM   #17
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from all thje reading ive done, airing UNDER RECOMMENDED Placard pressure results in Lowered MPG's.. tires often fall below this pressure as people air them exacxtly to placard pressure.. usually on mild weather days.. then when cold weather or raimy weather falls, the tires are run low..

running tires at ABOVE placard pressures seems not to net much in MPGs... im going on real world testing people have done.. that maybe they see a 1-2% MAX improvement... however most tests will have an errata of at least that due to wind, engine efficiencies in various temp / humidity conditions etc that vary..

the only advantage running a few over placard pressure seems to give is that your tires even when cold are still Above the recommended pressures...

if your placard is missing you can probably get the correct settings from your bus manufacturer if they are strill in Business.. I havent looked to see if the likes of bluebird vantage has the pressures listed or not..

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Old 04-17-2017, 09:32 PM   #18
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Quote:
Originally Posted by cadillackid View Post
from all thje reading ive done, airing UNDER RECOMMENDED Placard pressure results in Lowered MPG's.. tires often fall below this pressure as people air them exacxtly to placard pressure.. usually on mild weather days.. then when cold weather or raimy weather falls, the tires are run low..

running tires at ABOVE placard pressures seems not to net much in MPGs... im going on real world testing people have done.. that maybe they see a 1-2% MAX improvement... however most tests will have an errata of at least that due to wind, engine efficiencies in various temp / humidity conditions etc that vary..

the only advantage running a few over placard pressure seems to give is that your tires even when cold are still Above the recommended pressures...

if your placard is missing you can probably get the correct settings from your bus manufacturer if they are strill in Business.. I havent looked to see if the likes of bluebird vantage has the pressures listed or not..

-Christopher
I know hyper-milers like to run them over pressure. They also like to accelerate very slowly and coast wherever possible. For the very minimal improvement I would see from running over inflated tires, I will pass. We have tires that cost the same as 4 Prius tires for 1 steer tire.

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Old 04-17-2017, 09:39 PM   #19
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Thanks, I've got pdf information on hand for accuracy. My tire guys set these at 105 if I remember right. It seems good, but if I'm on a long trip I'd like to try out higher and lower pressures within a safety margin. At present the only reason I might lower my tire pressure for is if I was stuck at the beach. And no, I don't drive my bus on the beach.
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Old 04-17-2017, 09:50 PM   #20
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Quote:
Originally Posted by cadillackid View Post
air changing can be tough... esp if you do it within the same day due to temperature..

for instance here in Cbus its gonna be 45 tonight and then 75 tomorrow and sunny...

at 2% per 10 degrees F.. I could start with a tire that has 100 in the AM then has 106 by afternoon... then I drive the bus and heat it up to a tire temp of say 105 on the strong spring-sun drenched asphalt.. now im at 112... or a little higher since we deal in %... 12 PSI is enough to change the ride quality.... and if I was adjusting in the middle of the day without regards to temperature you can see where that goes.... I could quickly have an under-inflated 'COLD' tire right easily....

the pavement temperatures in Florida this time of year vary wildly also with somewhat cool air in place at night and that very stromng sun by day.
-Christopher
Ten four I know all about air pressure. I only check em cold.
But fwiw here in FL I RARELY find much variance in tire pressures from ambient temps. We don't have such extremes like you yanks. They were all at 85 psi last week, and 85psi this morning at 9. I went out to Shauns to use his compressor, and when I checked em hot at his house they were all reading about 5 psi higher. We washed and shined the tires, smoked out a bit, and I spent an hour or so in that fancy massage chair he's got in his garage.
After all that they were cooled back down and all reading 85psi again, so I made my adjustments. Seems to ride a little better, but like I said- could just be in my head.
I also only use a really good tire gauge. I'm anal about tire pressure, especially since owning an awd car.
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